1. Field of the Invention
This invention relates in general to a metal oxide nanoparticle sol and a method for preparing the same. More particularly, the invention relates to a zirconia nanocrystal sol and a method for preparing the same.
2. Description of the Related Art
The incorporation of zirconia nanoparticles into organic matrix materials (e.g., polymers) can increase the refractive index of optically transparent materials. The degree to which the refractive index of the organic matrix may be increased is a function of the loading limit of the nanoparticles in the organic matrix and the refractive index of the zirconia nanoparticles.
The nature of the zirconia (e.g., crystallinity, crystal structure, particle size and degree of primary particle association) governs the optical transmission, refractive index and the loading limit of the nanoparticles in an organic polymer. For a material to remain optical transparent after the incorporation of nanoparticles, the size of the nanoparticles has to be smaller than ¼ of the wavelength of the incident light, so that light is not scattered. Therefore the requirement of transparency posts an upper limit to the size of the primary and the associated particles. To increase the refractive index of the matrix, the refractive index of the incorporated nanoparticles must be higher than the matrix. In this respect, crystalline zirconia has a higher reflective index than non-crystalline zirconia, and is thus preferred. The refractive index of the matrix can be increased substantially if the loading of zirconia is high. However, the loading limit of a zirconia in a polymer is a function of both particle association and particle size. As particle size increases and/or the association between particles increases, the loading limit of the zirconia in a polymer decreases. Therefore, a method for preparing a sol containing nano-crystalline zirconia without association or having only weak association is important and necessary.
A stable sol containing non- or weakly-associated zirconia nanocrystals is in general transparent to visible light. However, the pH range where the sol remains stable should be as close to neutral condition as possible, preferably in the range of pH 6˜9, for easier application in various fields.
In a prior art, U.S. Pat. No. 4,784,794, a method of preparing a translucent zirconia sol by treating zirconium chloride at 130° C. or higher is disclosed, but the pH value of the prepared product is lower than 7. Additionally, U.S. Pat. No. 6,376,590 discloses a method of preparing a zirconia sol having high dispersing zirconia crystals by using a zirconium salt solution containing polyether or an organic acid having ether group therein as the starting material. However, the method requires high (140˜250° C.) temperature and the pH value of the prepared product is lower than 7. Since the process temperature described in the above methods would require the employment of a high-pressure autoclave, a low temperature process for the preparation of zirconia sol is needed.